Interior Calculators.
Paint, paper, board and floor — 4 tools, all free.
Room measurements turned into the things a finish job actually consumes: gallons and quarts of paint, single rolls of paper with the pattern repeat allowed for, sealed boxes of flooring, and sheets of drywall with the screws, tape and compound that always get left off the list.
Paint
Gallons and quarts for any room
Answer in gallonsWallpaper
Rolls with pattern waste
Answer in rollsFlooring
Boxes with waste built in
Answer in boxesDrywall
Sheets, screws, mud and tape
Answer in sheetsThe reasoning behind them
Guides that cover the judgement calls these tools can't make for you — what to measure, which assumption to change, and where the money goes when an estimate is wrong.
The rest of the wall
Outdoor & Yard Calculators, Electrical Calculators, Heating & Cooling Calculators — or all 18 in one list.